It has long been known that some materials are black in the visible spectrum but reflect or transmit to a considerable extent in the longer infrared regions. This has led to the application of black surfaces to obtain heat from solar radiation. Yet for flat-plate collectors without optical concentration devices, temperature is rather limited since heat losses from the receiver at higher temperatures, primarily from thermal radiation of the black surface, soon equal the incoming energy. Efforts have been directed to find or synthesize selective black surfaces so that they differentiate in their absorption, reflection or transmission characteristics between wavelengths above about 2 microns, i.e., in the "thermal" range and wavelengths below about 2 microns, i.e., in the "solar" range.
Polished zinc is an example of a natural surface with a fair degree of selectivity. For solar radiation the absorptance is of the order of 0.5. The emittance is about 0.05. However, polished zinc is a poor surface for a solar receiver since an absorptance of around 0.9-1.0 is needed. Consequently, efforts have been directed to synthesizing selective black surfaces for solar receivers. In general, materials having low emissivity in the infrared range are the metals -- the higher the electrical conductivity and the surface smoothness the lower the emissivity. Non-metals have a high emissivity unless they are transparent to long-wave radiation. Total emissivity for various materials is given in McAdams "Heat Transmission" -- McGraw-Hill Book Company, 1942, pp. 393-396.
A number of selective surfaces have been described in the art. These selective black surfaces generally are synthesized by taking a polished metal base and coating it with a very thin optically dark surface layer which is substantially transparent for wavelengths above about 2-3 microns. Exemplary are silver oxide on silver, iron oxide on steel, copper oxide on aluminum, copper oxide on copper and other metals, and nickel-zinc-sulfide complex, known as "nickel-black", on nickel, galvanized iron, or on other metals. The nickel-black and copper oxide coatings are generally used in practice. Preparation of a number of selective blacks is described by Tabor. U.S. Pat. No. 2,917,817, issued Dec. 22, 1959. The very thin layer of dark material can be obtained by such techniques as vapor deposition or electro deposition. An alternative method is to paint the dark material onto the metal surface; but this method, while inexpensive to do, suffers a disadvantage in that the thickness of the final layer is generally greater than that which can be obtained by the aforesaid deposition methods.
In nearly all the conventional surfaces there is considerable difficulty in getting a high solar absorptivity since as efforts are made to increase absorptivity, the emissivity usually rises very rapidly. An absorptance of 0.8 can often be obtained with almost no effect on the emittance of the base; however, by the time absorptivity has been raised to 0.9, by using thicker layers or changes in the recipe, the emittance has risen considerably.